Early in the development of integrated circuits, bipolar junction transistors (BJTs) were manufactured in a diffused BJT configuration that was not self-aligned. A device has no self-aligned structures, layers, or formations when the structures, layers and formations are formed with no precise adjustment or correct relative position to each other. In other words, each structure, layer, and formation of the device is formed independent of the position of the other existing structures, layers, and formations of the device.
In order to form the diffused BJT, a collector electrode is commonly first formed as a diffusion well having a first conductivity type. The collector electrode lies within a substrate and is exposed at a surface of the substrate. A base electrode is formed as a diffusion having a second conductivity type. The base electrode lies within the collector electrode diffusion well and is exposed at the surface of the substrate. In general, the base electrode diffusion is completely surrounded and electrically isolated from the substrate by the collector electrode diffusion well. An emitter electrode is formed as a diffusion well and is contained and electrically isolated within the base electrode diffusion. Electrical contact, via a conducting layer, is made to each of the emitter, base, and collector electrodes thereby completing the diffused BJT structure.
Due to heat cycles and the fact some diffusions in the diffused BJT structure are contained within other diffusions, diffusions in the diffused BJT structure tend to be relatively deep when compared to ideal junction depths. Deep junctions are usually not desired due to leakage currents, speed reduction, parasitic capacitance, and other known deep junction effects. A high series resistance can result in diffused BJTs, which degrades both amplification and switching performance. Diffused BJTs are also difficult to scale and diffusion wells are difficult to position and process consistently with respect to one another. The scaling, positioning, and processing problems result in devices, manufactured by the same process, that vary greatly in performance. In addition, the diffused BJT typically has a current carrying capability that is not as high as desired.
In order to increase amplification gain and improve upon the scaling problem, BJTs are formed with an emitter electrode which is doped via an overlying polysilicon layer. Doping via an overlying polysilicon layer allows diffusion junctions to be relatively shallow. Although this single-polysilicon BJT process results in improved performance over the diffused BJT, the single-polysilicon BJT has several of the diffused BJT disadvantages. Some examples being deep diffusions for the base and collector and a high series resistance.
To improve upon the single-polysilicon BJT, a double-polysilicon BJT is used. The double-polysilicon BJT uses a first layer of polysilicon for forming a base electrode and a second layer of polysilicon for forming an emitter electrode. Performance improves for the double-polysilicon BJT when compared to the single-polysilicon BJT. Due to a presence of exposed silicon regions, etch processing of the double-polysilicon BJT results in substrate trenching problems which leads to etch damage which results in leakage current and may affect series resistance. Also, a physically large base region results which creates larger capacitance and therefore slows the operation of the double-polysilicon BJT.